Witson— The Nebula surrounding Nova Perset. OOF 
was soon seen that either the Nova must be comparatively close to 
the Earth, or else that the velocity by which the nebula was ex- 
panding must be enormous. If we assume that the Nova is as 
close to us as the nearest fixed star, the velocity necessary to 
account for this apparent expansion must be about 2000 miles per 
second. Such velocities are quite unknown and most improbable. 
Early iast month the idea occurred to me that this apparent 
expansion might be due to the illumination of the solid particles 
of the nebula by the light sent out on the occasion of the outburst 
of the star, and that if this hypothesis were correct it was possible 
to calculate the distance of the star from the Harth by means of 
the observed angular growth of the illuminated ring which must 
spread out with the velocity of light. 
I have since found that Professor Kapteyn has quite inde- 
pendently suggested the same idea, and he can undoubtedly claim 
priority in its publication. 
Let D denote the distance of the Nova, and let L be the distance 
travelled over by light in a year of 365-25 days, #.e. a light year. 
Let T be the time in days elapsed from the outbreak of the star 
to the date of the photograph, and let p be the radius of arc in 
seconds from the Nova to the edge of the nebula, then 
D 206265 T 
i 6On@s ) o: 
ae D = [2:75184] x 2 L, 
© 
. : 206265 
the figure in brackets being the logarithm of 365-95 
The angular distance p of the point marked (A) in the photo- 
graph of September 20th is almost exactly 480”, and the time 
from the outbreak is 211 days. This makes the distance D of the 
Nova from the Earth 248 light years, or 15,780,000 times the 
distance from us of the Sun. 
Ii the Sun were removed to this distance its light would be re- 
duced to that of a star of the 10:24 magnitude, and would be, 
therefore, quite invisible, except in a telescope of considerable 
size. The brilliancy of the Nova at its maximum must have 
been extraordinary. As it appeared to us brighter than a Ist 
